Infrared optical systems having optical interference filters are often utilized for gas concentration measurements of anesthesia gases such as inhalation anesthetics, carbon dioxide and nitrous oxide. German Patent 196 28 310 C2 discloses a gas analyzer comprising a radiation source, a measuring path accommodating the gas sample and a detector which is connected to an evaluation unit. With the aid of the rotating filter wheel, different interference filters are sequentially introduced into the beam path and the wavelengths of these filters are matched to the absorption wavelengths of the gas components to be detected. For measuring inhalation anesthetics, the transmission wavelengths of the filters lie in a range between 8 micrometers and 12 micrometers; whereas, for nitrous oxide and carbon dioxide, the transmission wavelengths lie in the range between 3.5 micrometers and 4.5 micrometers. Eight interference filters are mounted on the filter wheel in the known gas analyzer. Furthermore, an evaluation method is provided for identifying inhalation anesthetics.
Drift effects can be compensated with reasonable complexity because the gas concentration measurement is carried out with only a radiation source and a detector. However, the measurement method with the use of movable parts is very cost intensive and subjected to wear. A measurement system of this kind cannot be miniaturized because of the number of interference filters to be accommodated on the filter wheel. Since the interference filters are always assigned to specific transmission wavelengths of the gas components to be detected, additional hydrocarbons such as alcohol or methane, which also can be contained in the breathing gas, cannot be detected without additional interference filters. These hydrocarbons affect the gas concentration measurements of the remaining anesthesia gas constituents.
A variable interferometer arrangement on the basis of a Fabry Perot interferometer affords certain advantages compared to an infrared absorption measurement with the wavelengths fixed by interference filters. A Fabry Perot interferometer of this kind comprises two partially transmissive mirrors which are aligned parallel with respect to each other and whose spacing to each other is changed. With the change of the mirror spacing, the possibility is afforded to evaluate a defined spectral range. Such interferometers can be manufactured cost effectively in miniaturized form because of silicon thin film technology. An interferometer of the above kind is described in U.S. Pat. No. 5,561,523 which is incorporated herein by reference. However, the wavelength range, which is to be evaluated for the gas concentration measurement of anesthesia gases, cannot be covered with the known interferometer.